Burner Assembly with Enhanced Flexibility

ABSTRACT

The invention relates to a burner assembly with enhanced flexibility, comprising a plurality of fuel rods that can be mounted in and removed from a fuel rod passage and that are connected to a fuel supply device by a flexible line, wherein each flexible line can be individually opened and closed.

The present invention relates to a burner assembly with enhancedflexibility and in particular such a dual-fuel burner and/or a burnerwith staged combustion.

It is well-known practice to use burners, and notably burners withstaged combustion, for the heating of a load in a furnace, such as amelting furnace.

Regular maintenance of the burner is essential for it to operatecorrectly. The maintenance of a burner usually includes the inspectionof the state of the injectors or fuel rods, their cleaning or theirreplacement.

Such maintenance normally requires taking the burner out of service forthe duration of the maintenance procedure.

In the case of a furnace furnished with a single burner, taking theburner out of service corresponds to stopping the heating of thefurnace.

In the case of a furnace furnished with several burners, compensatingfor the loss of power due to taking one of the burners out of servicefor its maintenance by a corresponding increase in the power of theother burners of the furnace can in theory be envisaged. However, such aprocedure causes a change in the thermal profile of the furnace whilethe burner is out of service.

It is evident that controlling the range of temperatures in anindustrial furnace is essential for the performance of the furnace andfor the quality of the product coming out of the furnace.

The result of the foregoing is that, because the maintenance of a burnercauses it to be taken out of service, the operation of maintaining theburner or burners of an industrial furnace has a temporary negativeimpact on the performance of the furnace and/or the quality of theproduct. Moreover, the maintenance of the burner or burners is essentialfor the safety of the furnace and its longer-term productivity.

There is therefore a need to be able at least to limit the downtime of aburner in an industrial furnace when it is being maintained.

Another important problem which the operators of industrial furnaceshave to face is the instability of the price of the fuels, such asnatural gas.

In response, the operators of industrial furnaces wish to be able totake out intermittent fuel-supply contracts. This results in a lowercost of fuel; but, on the other hand, the distributors can temporarilystop the supply. The clients are warned in advance of the stoppage ofsupply and the contracts are negotiated so that the shorter thenotification time, the lower the negotiated fuel cost.

The operators of an industrial furnace must then be capable of changingthe type of fuel, for example switching natural gas with liquid fuel, ina very short time.

Switching a burner from a first fuel to a second fuel usually requiresthe fuel injectors to be replaced and therefore normally the burner tobe taken out of service. This operation may take several hours in thecase of cold reagents, but it may be spread over several days in thecase of preheated reagents.

For the reasons indicated above with respect to the maintenance of theburners, there is a need to be able to limit the downtime of a burner inan industrial furnace when there is such a change from one fuel toanother.

The present invention proposes a burner assembly which makes it possibleto maintain the fuel rods without having to stop the burner, and evenwithout having to reduce the power of the burner.

The present invention also proposes a burner assembly which allows theswitch from a first fuel to a second fuel or to a combination of severalfuels and vice versa without having to stop the burner, and even withouthaving to reduce the power of the burner.

The present invention relates more particularly to a burner assemblycomprising:

-   -   a burner unit,    -   n fuel rods or fuel injectors called first-fuel rods, where n>1,    -   an oxidant-supply device and    -   a first fuel-supply device.

According to the invention, the burner unit has an inlet face and anoutlet face, at least one oxidant passageway between the inlet face andthe outlet face and at least one fuel-rod passageway between the inletface and the outlet face. Said burner unit is such that m fuel rods canbe simultaneously mounted in the burner unit through the at least onefuel-rod passageway, where m>1.

Each of the first-fuel rods is capable of being mounted in the or in oneof the fuel-rod passageways, and of being removed from said fuel-rodpassageway via the inlet face of the burner unit. For the maintenance ofsaid rods, it is specifically important to be able to insert and removethe rods without the rods, the burner unit and in particular thefuel-rod passageway(s) being damaged thereby.

The oxidant-supply device is capable of transporting a flow of oxidantfrom a source of oxidant to the burner unit for its injection throughthe at least one oxidant passageway into a combustion zone situateddownstream of the outlet face.

Similarly, the first fuel-supply device is capable of transporting aflow of a first fuel from a first-fuel source to the burner unit for itsinjection into the combustion zone through one or the fuel-rodpassageway(s). This first fuel-supply device comprises a first supplyline, a first flow meter, a first distributor and n first flexible linesconnecting the first distributor to the n first-fuel rods. The firstsupply line is more particularly capable of transporting said flow ofthe first fuel from the first-fuel source to the first distributor. Thefirst flow meter is capable of regulating the rate of flow of the firstfuel from the first-fuel source to the first distributor and the firstdistributor is capable of dividing the flow of first fuel into nsubsidiary flows on the n first flexible lines.

The flexible lines make it much easier to insert the fuel rods into thefuel-rod passageway(s) and to remove the fuel rods from saidpassageways.

The burner assembly according to the present invention is characterizednotably in that the first fuel supply device comprises one or morevalves making it possible to close or open the n first flexible linesone by one so that, when x of the n first flexible lines are closed bysaid one or more valves, where 1≦x≦n−1, the first distributor dividesthe flow of first fuel into the n−x first flexible lines which are openfor its injection into the combustion zone by the first-fuel rod or rodsconnected to said first open flexible lines.

The burner assembly according to the invention therefore makes itpossible, during the maintenance of one or, if necessary, several of itsfirst-fuel rods, to keep the burner assembly in operation and to do sowithout having to reduce the level of the power supplied by said burnerassembly to the combustion zone. In this manner, the effect of themaintenance of the first-fuel rods on the thermal profile in thefurnace, and therefore of the productivity of the furnace and/or thequality of the product originating from the furnace, is greatly reduced.

Clearly, when the number x of first flexible lines closed by said one ormore valves is equal to n−1, the totality of the flow of first fuel issent into the only first flexible line left open by the firstdistributor. When all the first flexible lines are closed, no flow offirst fuel is injected into the combustion zone.

Each first flexible line may, for example, be furnished with a valvemaking it possible to open and close this flexible line. The firstfuel-supply device may also comprise a valve which makes it possible toclose and open selectively one or more of the first flexible lines.

The burner assembly may comprise a central control unit for the openingand the closure of the first flexible lines by the valve(s) of the firstfuel-supply device and/or means for the manual operation of this orthese valves.

In the present context, it is considered that a flexible line is closedwhen it does not allow a flow of fuel to pass and that a flexible lineis open when it allows a fuel flow to pass through the flexible line.

The first fuel may in particular be a gaseous fuel (such as natural gas)or a liquid fuel, such as fuel oil.

According to one embodiment, the number n of first-fuel rods is equal tothe number m of fuel-rod passageways in the burner unit.

According to an alternative embodiment, the number n of first-fuel rodsis equal to the number m of fuel-rod passageways+1.

Therefore, for a burner assembly having, in normal operation, aninjection of first fuel into the combustion zone by means of mfirst-fuel rods, this embodiment makes it possible, when one of thefirst-fuel rods is taken out of service for its maintenance by theclosure of the corresponding first flexible line, to replace in theburner unit the rod under maintenance with the m+1^(st) first-fuel rodand to open the first flexible line that corresponds to this m+1^(st)rod so as to be able to operate the whole burner with m first-fuel rodsduring the maintenance of one of the first-fuel rods. In this manner,the effect of the maintenance of the first-fuel rods on the thermalprofile in the furnace, and therefore on the productivity of the furnaceand/or the quality of the product coming from the furnace, is again moreclearly reduced, or even eliminated.

It is also possible to provide a source of fuel called a reserve source,linked by means of a reserve-fuel supply device to a reserve-fuel rod soas to be able, for a burner assembly having, in normal operation, aninjection of first fuel into the combustion zone by means of mfirst-fuel rods, when one of the m first-fuel rods is taken out ofservice for its maintenance by the closure of the corresponding firstflexible line, to replace in the burner unit the first fuel-rod undermaintenance with this reserve-fuel rod and thus be able to operate theburner assembly with m⁻¹ first-fuel rods and the reserve-fuel rod duringthe maintenance of one of the first-fuel rods. Such an operating mode isparticularly recommended in the case of a furnace or other industrialinstallation comprising a considerable number of burner assemblies.

The reserve fuel may also be a gaseous or liquid fuel. The reserve fuelis advantageously a liquid fuel, notably because of the ease of storageand transport of such a fuel.

The reserve fuel may be of the same type (gaseous, liquid, etc.) as thefirst fuel or of a different type. The composition of the reserve fuelmay match or be different from the composition of the first fuel.

The present invention also relates to a burner assembly as describedabove and which also comprises:

-   -   p second-fuel rods, where p>1 and    -   a second fuel-supply device.

Each of said second-fuel rods is able to be mounted in the or in one ofthe fuel-rod passageways and to be removed from said fuel-rod passagewayvia the inlet face, as already indicated above for the first-fuel rods.

The second fuel device is capable of transporting a flow of asecond-fuel from a second one or fuel source to the burner unit for itsinjection into the combustion zone through one or the fuelpassageway(s). The second fuel-supply device comprises a second supplyline, a second flow meter, a second distributor and p second flexiblelines connecting the second distributor to the p second-fuel rods.

The second supply line is capable of transporting the flow of the secondfuel from the second-fuel source to the second distributor. The secondflow meter is capable of regulating the rate of flow of the second-fuelfrom the second-fuel source to the second distributor. The seconddistributor is capable of dividing the flow of second fuel into psubsidiary flows on the p second flexible lines.

According to this embodiment of the invention, the second fuel-supplydevice comprises one or more valves making it possible to close and openthe p second flexible lines one by one so that, when y of the p secondflexible lines are closed by said one or more valves, where 1≦y≦p−1, thesecond distributor divides the flow of second fuel over the p−y secondflexible line(s) which is/are open for its injection into the combustionzone by the second-fuel rod or rods connected to said second openflexible line(s).

Each second flexible line can, for example, be furnished with a valvemaking it possible to open and to close this flexible line. The secondfuel-supply device may also comprise a valve which makes it possible toclose and to open selectively one or more of the second flexible lines.

The burner assembly may comprise a central control unit for opening andclosing the second flexible lines by one or the valves of the secondfuel-supply device and/or means for the manual operation of this orthese valves.

The second fuel may in particular be a gaseous fuel or a liquid fuel.

The second fuel may be of the same type (gaseous, liquid, etc.) as thefirst fuel, but in practice most frequently of a different type.

When use is made of a reserve fuel, the reserve fuel may also be agaseous or liquid fuel. The reserve fuel is advantageously a liquidfuel, such as fuel oil, notably because of the ease of storage and oftransporting such a fuel.

The number n first-fuel rods may be equal to the number p second-fuelrods.

The number p of second-fuel rods may be equal to the number m offuel-rod passageways of the burner unit.

As already described above with reference to the first-fuel rods, thenumber p of second-fuel rods may be equal to the number m+1 of fuel-rodpassageways.

Burner assemblies comprising 2 passageways of fuel (m=2) have been foundto be particularly practical for a number of applications, and notablyfor melting furnaces.

The burner assembly according to the invention may be used in aircombustion, but is particularly useful for applications inoxycombustion. The invention therefore relates in particular to a burnerassembly in which the source of oxidant is a source of oxidant having anoxygen content of at least 80% vol, preferably of at least 90% vol.

The burner assembly according to the invention is notably advantageousfor staged combustion.

The method of staged combustion of fuels consists in dividing thequantity of oxidant necessary for the total combustion of the fuel intoat least two flows of oxidant inserted into the combustion zone atdifferent distances from the injection of the flow or flows of fuel intothis combustion zone. Therefore, at least one first flow of oxidant isinjected very close to or together with the flow or flows of fuel. Theoxidant injected by this or these flows of injected oxidant closest toor with the flow or flows of fuel is called primary oxidant. Notably itallows the partial combustion of the fuel at a controlled temperaturewhich limits for example the formation of NOx. The or the other flow(s)of oxidant are injected into the combustion zone at a greater distancefrom the fuel than the primary oxidant. The oxidant thus injected makesit possible to achieve the combustion of the fuel that has not reactedwith the primary oxidant. The oxidant injected by this or these latterflows is called secondary oxidant.

Document WO 02/081967 describes a method making it possible to applythis type of staged combustion method. The oxidant is separated intothree distinct flows which are injected at different distances from theinjection point of the fuel and at different speeds. Therefore, a firstjet of oxidant is injected at a high speed at the center of the jet offuel. Then, a second jet of oxidant is injected at a lower speed at afirst distance from the jet of fuel. Finally, a third jet of oxidant isinjected at a second distance from the jet of fuel, this second distancebeing greater than the first distance.

The invention therefore also relates to a burner assembly in which theoxidant-supply device is capable of transporting several oxidant flowsfrom a source of oxidant to the burner unit for its injection throughthe at least one oxidant passageway into the combustion zone, at leastone of said oxidant flows being a secondary oxidant flow injected intothe combustion zone through an oxidant passageway situated at a distanceds>0 from the fuel-rod passageway(s) and at least one of said oxidantflows being a primary oxidant flow injected into the combustion zonethrough the or one of the fuel rod passageways or through an oxidantpassageway situated at a distance dp from the fuel-rod passageway(s),where 0≦dp<ds.

According to one embodiment making it possible to stabilize the flame asmuch as possible in the combustion zone, at least one of the oxidantflows is a primary oxidant flow injected through the or one of thefuel-rod passageways around one or the fuel rod(s).

It is known practice to use an oxidant at a high temperature, namely atemperature of at least 100° C., or even of several hundreds of degreesin order to improve the energy efficiency of the combustion.

The inventors have in particular proposed, in application EP-A-1995543,heat exchangers making it possible to heat the oxygen satisfactorily.

The conveyance of a hot oxidant requires the use of specific means,usually fixed means, which do not lend themselves to the frequentmaintenance operations mentioned above.

According to one embodiment of the invention for staged combustion, theoxidant-supply device comprises means for the heating of the at leastone secondary oxidant flow to a temperature of at least 100° C. upstreamof the burner unit, said oxidant-supply device comprising no means forthe heating of the at least one primary oxidant flow. Said primaryoxidant, which is injected into the combustion zone through the or oneof the oxidant-rod passageway(s) or through one or the oxidantpassageway(s) situated at a distance dp from the fuel-rod passageway(s)is therefore not heated or preheated to a temperature of 100° C. or moreat the inlet of the burner unit.

In the present context, the terms “heated” and “preheated” are usedinterchangeably.

Preferably, the secondary oxidant is preheated to a temperature at theinlet of the burner unit of between 100° C. and 650° C., preferablybetween 100° C. and 600° C., and again preferably between 350° C. and550° C.

This embodiment makes it possible to improve energy efficiency throughthe use of a preheated oxidant for the secondary oxidant, whileretaining the enhanced flexibility of the burner according to theinvention, notably with respect to the maintenance of the fuel rodsand/or the switch from a first fuel to a second fuel or to a combinationof several fuels, through the use of a non-preheated oxidant for theprimary oxidant.

For the system to have the advantage of the energy balance inherent inthe use of a preheated oxidant, the non-preheated primary oxidantprovides only a very small fraction of all the oxidant necessary for thetotal combustion of the fuel. This fraction is usefully less than 10% ofall the oxidant. Advantageously, this proportion ranges between 1.5 and7% of all the oxidant necessary to ensure the complete combustion of thefuel.

It is understood that the benefit of using a hot oxidant is notsubstantially affected by this very small fraction of unheated oxidant.

By unheated or non-(pre)heated, it should be understood that the(primary) oxidant is in the essential ambient temperature conditionsthat prevail on its path taking it to the furnace. On passing therefractory walls of the furnace, its temperature necessarily rises. Thetemperature of the non-(pre)heated oxidant is preferably the ambienttemperature and must not exceed one hundred degrees Celsius, thetemperature in the vicinity of the furnace nevertheless beingsubstantially higher than that of the atmosphere at a distance from thefurnace.

The primary oxidant and the secondary oxidant may have an identical ordifferent composition. When the primary oxidant and the secondaryoxidant have one and the same composition, they can be provided by oneand the same source of oxidant, such as, for example, a unit forseparating gases from the air, the secondary oxidant passing through,upstream of the burner unit, means for the preheating of said secondaryoxidant, such as notably heat exchangers, while the primary oxidant doesnot pass through any means for preheating it upstream of the burnerunit.

The burner unit may consist of a single brick. The burner unit may alsoadvantageously consist of a set of several bricks, typically firebricks. When the burner unit consists of several bricks, said bricks maybe spaced out, notably when the burner assembly is mounted in a wall ofa furnace. The use of a burner unit which consists of several bricksallows in particular a staging of the combustion that is more spacedout, that is to say the injection of at least one jet of secondaryoxidant at a greater distance from the injection(s) of fuel. A burnerunit which consists of several bricks may also allow a more spacedinjection of different jets of fuel.

As already indicated above, the invention relates notably to a burnerassembly in which the burner unit is mounted in a wall of a furnace, thecombustion zone downstream of the outlet face being situated inside thefurnace. The invention also relates to a furnace comprising such aburner assembly.

The invention also relates to the use of one or more of these burnerassemblies for carrying out the combustion of oxidant and fuel in acombustion zone and in particular in such a combustion zone inside afurnace, and any combustion method by means of a burner assembly or of afurnace according to the invention.

The invention relates particularly to the use of one or more burnerassemblies for carrying out staged combustion in the combustion zone andin particular staged combustion with one or more unheated primaryoxidant flows and one or more heated secondary oxidant flows.

EXAMPLES

The advantages of the present invention appear more clearly from thedetailed description given as an example below which refers to FIGS. 1to 6 in which:

FIG. 1 is a schematic representation of the operation of the fuel- andoxidant-supply devices of a staged-combustion burner assembly accordingto the invention,

FIGS. 2 and 3 are schematic representations of the operation of thefuel- and oxidant-supply devices of one embodiment of astaged-combustion burner assembly according to the invention with areserve rod of first fuel connected to the first distributor before andduring a maintenance procedure on a first-fuel rod,

FIG. 4 is a schematic representation of the operation of the fuel- andoxidant-supply devices of one embodiment of a staged-combustion burnerassembly according to the invention with a reserve rod connected to asource of reserve fuel,

FIGS. 5 and 6 are schematic representations of the operation of thefuel- and oxidant-supply devices of one embodiment of astaged-combustion burner assembly according to the invention during andafter the change from a first fuel to a second fuel.

In FIG. 1, the first supply line 100 of the first fuel-supply deviceconnects a source of first fuel (not illustrated) to the firstdistributor 101. The flow of first fuel from the source of first fuel tothe first distributor is regulated by a first flow meter (notillustrated).

The first distributor 101 is connected to the two first-fuel rods 110and 111 by the two first flexible lines 120 and 121 (n=2).

Each of the two first-fuel rods 110 and 111 is inside a fuel-rodpassageway 500, 501 between the inlet face and the outlet face 503 ofthe unit (m=2).

The first fuel-supply device also comprises two valves 130 and 131making it possible to close and open the first flexible lines 120 and121 individually.

The second supply line 200 of the second fuel-supply device connects asource of second fuel (not illustrated) to the second distributor 201.The second supply device comprises a second flow meter (not illustrated)for the regulation of a flow of second fuel from the source of secondfuel to the second distributor. The second supply device also comprisestwo second flexible lines 220 and 221 in order to connect the seconddistributor to two second-fuel rods (not illustrated) (p=2), each secondflexible line being able to be opened and closed by the valves 230 and231.

The oxidant-supply device comprises secondary oxidant-supply lines 504for transporting secondary oxidant to two secondary-oxidant passageways506, 507 situated at a distance ds from the fuel passageways 110,respectively 111.

The oxidant-supply device also comprises a primary oxidant supplysystem. In the embodiment illustrated in the figures, the primaryoxidant supply system comprises a primary oxidant supply line 508 and aprimary oxidant distributor 509. The primary oxidant distributor 509 isconnected to the fuel passageways 500 and 501 by pipes 510, 511. Theprimary oxidant is injected into the combustion zone 512 through anannular zone around the fuel rods 110 and 111 in the fuel passageways500 and 501 (dp=0). Primary oxidant thus injected around a fuel rod isusually called “sheathing oxidant”. The two valves 530 and 531 make itpossible to open and close the two primary oxidant pipes 510 and 511.

In order to change the type of fuel or to carry out maintenance of oneof the m primary fuel rods (in this instance m=2), while keeping theburner alight and at a constant power, the following steps must becarried out:

-   -   (a) Close the first flexible line 120 (121) corresponding to the        targeted first-fuel rod 110 (111). This closure is achieved by        closing the valve 130 (131). The totality of the flow of first        fuel, for example of natural gas, is then automatically diverted        to the m−1 (in this instance m−1=1) first flexible lines 121        (120) that are still open and therefore to the corresponding m−1        first-fuel rods 131 (130). The power of the burner remains the        same.    -   (b) Stop the flow of sheathing oxidant around the targeted        first-fuel rod 120 (121) in order to be able to remove this rod        from the fuel-rod passageway 500 (501) by closing the pipe of        primary oxidant 510 (511) by means of the valve 530 (531). The        totality of the flow of sheathing oxidant is then diverted to        the fuel passageways 501 (500) containing the m−1 fuel rods 121        (120) that are still in operation (in this instance m−1=1).    -   (c) Remove the stopped first-fuel rod and optionally carry out        the necessary maintenance.

In the case of a purely maintenance operation, after the maintenance orthe replacement of the first-fuel rod in question, the reverse procedureis carried out: the stopped first-fuel rod 110 (111) is reinserted intoits fuel-rod passageway 500 (501); reopen the flow of sheathing oxidantaround this first-fuel rod 110 (111) in this fuel-rod passageway 500(501) by opening the primary-oxidant pipe 510 (511) by means of thevalve 530 (531) and open the first flexible line 120 (121) correspondingto this first-fuel rod 110 (111) by means of the valve 130 (131). Thetotality of the flow of first fuel is then automatically redistributedto the m (in this instance m=2) first flexible lines 121 and 120 andtherefore to the m first-fuel rods 131 and 130.

Following this example, the burner assembly operates with m−1 first-fuelrods during the maintenance of one of the first-fuel rods. The power ofthe burner can remain constant during the maintenance procedure and theimpact on the productivity of the furnace and/or on the quality ofproduct coming out of the furnace is greatly limited.

The burner assembly illustrated in FIG. 2 makes it possible to furtherlimit the impact of a maintenance procedure or change of fuel rods onthe productivity of the furnace and/or on the quality of the productcoming out of the furnace, notably if the operator wishes or is obligedto carry out long-term maintenance.

The first fuel-supply device of the burner assembly according to FIG. 2comprises one m+1^(st) first-fuel rod 112, one m+1^(st) first flexibleline 122 connected to the first distributor 101, and one m+1^(st) valve132 making it possible to open and close the m+1^(st) first flexibleline 122 (in this instance m+1=3).

Before the maintenance procedure, the first flexible line 122 is closedand the first-fuel rod 112 is out of service.

During the maintenance of, for example, the first-fuel rod 110, theoperator may, after step (c) above:

(d) replace the first-fuel rod 110 with a rod 112 of the same fuelpreviously connected to the first distributor 101 of said fuel,

-   -   (e) reopen the flow of sheathing oxidant in the fuel-rod        passageway 500, this time around the first-fuel rod 112 by        opening the pipe of primary oxidant 510 by means of the valve        530, and    -   (f) open the first flexible line 122 corresponding to this        first-fuel rod 112 by means of the valve 132.        As shown in FIG. 3, the totality of the flow of first fuel is        then automatically redistributed to the m (in this instance m=2)        first flexible lines 122 and 121 and therefore to the m        first-fuel rods 132 and 131. The maintenance of the rod 110 can        then be carried out while the burner assembly operates at normal        rate.

FIG. 4 shows an alternative embodiment of a burner assembly according tothe invention.

If the operator desires or is obliged to carry out long-term maintenanceof, for example, the first-fuel rod 110, the operator may, after step(c) above:

-   -   (g) replace the first-fuel rod 110 with a reserve-fuel rod 310        connected to a source of reserve fuel (not illustrated), such        as, for example, a source of fuel oil. The reserve fuel rod 310        is a rod specifically designed for the injection of said reserve        fuel,    -   (h) reopen the flow of sheathing oxidant in the fuel-rod        passageway 500, this time around the reserve-fuel rod 310 by        opening the primary-oxidant pipe 510 by means of the valve 530,        and    -   (i) open the reserve flexible line 320 connected to the        reserve-fuel rod by means of the valve 330 and regulate the flow        of reserve fuel to the reserve-fuel rod 310 and the flow of        first fuel to the m−1 first-fuel rods 111 connected to the m−1        first flexible lines 121 that are open (in this instance m−1=1)        so as to achieve the desired power for the burner assembly.

In the precise case of fuel oil, or of other liquid fuels, as a reservefuel, each fuel-oil rod 310 is also supplied by an atomization fluid. Aflexible line 420 must also therefore be provided for this atomizationfluid with its associated valve 430. The operator opens the valve 430before opening the fuel-oil valve 330, which in this case is the reservefuel. In order to achieve the nominal power, the operator reduces,manually or by programmable controllers, the power of the fuel 1 whileincreasing the power of the fuel 2 up to half of the burner power, allwhile complying with the flow-range setpoints of the atomization fluid.

As shown in FIG. 4, the burner assembly then operates at mixed rate withinjection into the combustion zone on the one hand of a flow of firstfuel and on the other hand of a flow of reserve fuel. The maintenance ofthe rod 110 can then be carried out while the burner assembly operatesat mixed rate with a minimal impact on the productivity of the furnaceand/or on the quality of the product coming out of the furnace.

In order to completely replace the first fuel with the second fuel inthe embodiment illustrated in FIG. 1, it is necessary, as illustrated inFIGS. 5 and 6, to follow a similar scenario for the replacement of the mfirst-fuel rods (in this instance m=2) with the p second-fuel rods (inthis instance m=2).

After step (c) above, the operator:

-   -   (j) replaces the first-fuel rod 110 with a second-fuel rod 210        connected to the second distributor 201 of the second fuel,    -   (k) reopens the flow of sheathing oxidant in the fuel-rod        passageway 500, this time around the second-fuel rod 210 by        opening the primary-oxidant pipe 510 by means of the valve 530,        and    -   (l) opens the second flexible line 220 corresponding to this        second-fuel rod 210 by means of the valve 230 while regulating        the flows of first and second fuels to the fuel rods 111 and 210        connected to the first and second open flexible lines 121 and        220 so as to achieve the desired power for the burner assembly.

The burner assembly then operates (FIG. 5) at mixed rate with injectioninto the combustion zone on the one hand of a flow of first fuel and onthe other hand of a flow of second fuel.

Steps (j) to (l) are then repeated for the replacement of the m−1 (inthis instance m−1=1) other first-fuel rods 111 with second-fuel rods 121in order to achieve an operation of the burner assembly with only oneflow of second fuel (FIG. 6) and at the desired power.

Therefore, the burner assembly according to the invention makes itpossible to switch from a first fuel to a second fuel, and even to amixed rate of two fuels, without having to stop the burner assembly andwithout having to reduce the power of the burner assembly and with aminimal impact on the productivity of the furnace and/or on the qualityof the product coming out of the furnace.

The above comments relating to the rods of liquid fuel clearly apply toall of the rods of liquid fuel which may be first-fuel rods, second-fuelrods or reserve rods.

1-15. (canceled)
 16. A burner assembly comprising: a burner unit havingan inlet face and an outlet face, at least one oxidant passagewaybetween the inlet face and the outlet face and at least one fuel-rodpassageway between the inlet face and the outlet face, the burner unitbeing such that m fuel rods can be simultaneously mounted in the burnerunit through the at least one fuel-rod passageway, where m>1, nfirst-fuel rods, where n>1, each first-fuel rod being capable of beingmounted in the or in one of the fuel-rod passageways, and of beingremoved from said fuel-rod passageway via the inlet face of the burnerunit, an oxidant-supply device capable of transporting a flow of oxidantfrom a source of oxidant to the burner unit for its injection throughthe at least one oxidant passageway into a combustion zone situateddownstream of the outlet face, a first fuel-supply device capable oftransporting a flow of a first fuel from a first-fuel source to theburner unit for its injection into the combustion zone through one orthe fuel passageway(s), the first fuel-supply device comprising a firstsupply line, a first flow meter, a first distributor and n firstflexible lines connecting the first distributor to the n first-fuelrods, i. the first supply line being capable of transporting the flow ofthe first fuel from the first-fuel source to the first distributor, ii.the first flow meter being capable of regulating the rate of flow of thefirst fuel from the first-fuel source to the first distributor, iii. thefirst distributor being capable of dividing the flow of first fuel inton subsidiary flows on the n first flexible lines, wherein: the firstfuel-supply device comprises one or more valves making it possible toclose or open the n first flexible lines one by one so that, when x ofthe n first flexible lines are closed by said one or more valves, where1≦x≦n−1, the first distributor divides the flow of first fuel into then−x first flexible lines which are open for its injection into thecombustion zone by the first-fuel rod or rods connected to said firstopen flexible lines.
 17. The burner assembly of claim 16, wherein n=m.18. The burner assembly of claim 16, wherein n=m+1.
 19. The burnerassembly of claim 16, further comprising: p second-fuel rods where p>1,each second fuel rod being able to be mounted in the or in one of thefuel-rod passageways and to be removed from said fuel-rod passageway viathe inlet face, a second fuel-supply device capable of transporting aflow of a second fuel from a second-fuel source to the burner unit forits injection into the combustion zone through one or the fuel-rodpassageway(s), the second fuel-supply device comprising a second supplyline, a second flow meter, a second distributor and p second flexiblelines connecting the second distributor to the p second-fuel rods, i.the second supply line being capable of transporting the flow of thesecond fuel from the second-fuel source to the second distributor, ii.the second flow meter being capable of regulating the rate of flow ofthe second fuel from the second-fuel source to the second distributor,iii. the second distributor being capable of dividing the flow of thesecond fuel into p subsidiary flows on the p second flexible lines,wherein: the second fuel-supply device comprises one or more valvesmaking it possible to close and open the p second flexible lines one byone so that, when y of the p second flexible lines are closed by saidone or more valves, where 1≦y≦p−1, the second distributor divides theflow of second fuel over the p−y second flexible line(s) which is/areopen for its injection into the combustion zone by the second-fuel rodor second-fuel rods connected to said second open flexible line(s). 20.The burner assembly of claim 19, wherein n=p.
 21. The burner assembly ofclaim 19, wherein p=m.
 22. The burner assembly of claim 19, whereinp=m+1.
 23. The burner assembly of claim 16, wherein m=2.
 24. The burnerassembly of claim 16, wherein the source of oxidant is a source ofoxidant having an oxygen content of at least 80% vol, preferably of atleast 90% vol.
 25. The burner assembly of claim 16 for the stagedcombustion, wherein: the oxidant-supply device is capable oftransporting several oxidant flows from a source of oxidant to theburner unit for its injection into the combustion zone, at least one ofsaid oxidant flows being a secondary oxidant flow injected into thecombustion zone through an oxidant passageway situated at a distanceds>0 from the fuel-rod passageway(s), and at least one of said oxidantflows being a primary oxidant flow injected into the combustion zonethrough the or one of the fuel-rod passageways or though an oxidantpassageway situated at a distance dp from the fuel-rod passageway(s),where dp<ds.
 26. The burner assembly of claim 25, wherein at least oneof the oxidant flows is a primary oxidant flow injected through the orone of the fuel-rod passageways and around one or the fuel rod(s). 27.The burner assembly of claim 16, wherein the burner unit is an assemblyof several fire bricks.
 28. The burner assembly of claim 16, wherein theburner unit is mounted in a furnace wall, the combustion zone downstreamof the outlet face being situated inside the furnace.
 29. A furnacecomprising the burner assembly of claim
 28. 30. A combustion method,comprising the step of combusting oxidant and fuel injected by theburner of the furnace of claim 29.